discovery: always add chan announcements to the reject cache if err != ErrIgnored
discovery: always add chan announcements to the reject cache if err != ErrIgnored

In this commit, we make a change to always add chan announcements to the
reject cache if we didn't reject them for already existing.  Without
this change, if we end up rejecting a channel announcement say because
the channel is already spent or the funding transaction doesn't exist,
then we'll end up continually re-validating the same set of channels we
know will fail, when they're sent to us by peers.

Fixes #5191

aakselrod/lnd,aakselrod/lnd